1. Field of the Invention
The present invention relates to a superconducting current limiting element instantaneously suppresses an excessive current, such as a short circuit current transmitting electric power transmission lines, utilizing transition of a superconductor from a superconducting state to a normal-conducting state.
2. Description of the Related Art
A current limiting element is capable of conducting a current having a certain value or less while hardly generating a loss, but, when a high current is to be flowed, generates impedance to suppress the current. The current limiting element is applied to electric power equipment required to suppress a fault current due to short circuit or ground fault occurred in an electric power system. At present, various types of current limiting elements have been studied such as an element using arc discharge, and an element utilizing a semiconductor. Among them, a current limiting element referred to as a resistive superconducting current limiting element is studied in many research organizations. The resistive superconducting current limiting element utilizes a phenomenon that, when a high current is to be flowed, the superconductor cannot retain the superconducting state and becomes to produce a resistance through superconducting-to-normal transition. The element has superior features of high-speed operation and a compact design.
However, a superconductor cannot be used as a current limiting element if it is simply connected in series in the electric power system, because it is easily damaged in this case. A detailed mechanism why the superconductor is damaged has not been known at present, but the following causes are considered. That is, in the process of superconducting-to-normal transition due to the high current, a normal-conducting region is locally formed in the superconductor because of inhomogeneity of superconducting characteristics. It is considered that, if a high current not suppressed yet flows through the local normal-conducting region immediately after a current limiting operation is started, a very large amount of Joule heat is generated, which damages the superconductor. In addition, following the local temperature rise, high internal thermal stress is generated in the case of a bulk superconductor, or high interfacial thermal stress acts in the case of a superconducting thin film formed on an insulating substrate. Such thermal stress is also considered as a cause of damaging the superconductor.
Therefore, to manufacture a repeatedly operable resistive superconducting current limiting element, there is required such a measure as to make the normal-conducting region formed uniformly within the superconductor when an overcurrent flows, or to reduce the Joule heat generated in the normal-conducting region in an initial stage of the current limiting operation. Of the two methods, the latter method can be realized comparatively easily by connecting a resistor formed of a normal conductor in parallel with the superconductor. Therefore, most of the resistive superconducting current limiting elements which have been studied at present adopt such a structure.
FIG. 1 shows a conventional resistive superconducting current limiting element in which a normal conductor 102 is connected in parallel with a superconductor 101. This figure shows a state where a normal-conducting region 101a is locally formed within the superconductor 101 in the initial stage of the current limiting operation. Here, r denotes the resistance value of the normal conductor 102, and R denotes the resistance value of the normal-conducting region 101a in the superconductor 101.
Now, assume that the normal conductor 102 is not connected in parallel with the superconductor 101; In this case, quantity of heat W generated in the normal-conducting region 101a locally formed within the superconductor 101 is expressed by the following equation (1) and, in general, the quantity of heat W is remarkably high.W=I2R  (1).
On the other hand, when the normal conductor 102 is connected in parallel with the superconductor 101 as shown in FIG. 1, a part of a current I is commuted to the normal conductor 102, and therefore the quantity of heat generated in the normal-conducting region 101a within the superconductor 101 can be reduced. However, the conventional resistive superconducting current limiting element shown in FIG. 1 also has a problem.
In the structure of FIG. 1, the quantity of heat W generated in the normal-conducting region 101a is expressed by the following equation (2):
                    W        =                                                            r                ⁢                                                                  ⁢                R                                            (                                  R                  +                  r                                )                                      ⁢            I            ×                          r                              (                                  R                  +                  r                                )                                      ⁢            I                    =                                                                      r                  2                                ⁢                R                                                              (                                      R                    +                    r                                    )                                2                                      ⁢                                          I                2                            .                                                          (        2        )            
In order to make the quantity of heat W represented by the equation (2) sufficiently lower than the quantity of heat W represented by the equation (1), it is necessary to satisfy the condition of the following equation (3):r<<R  (3).
Moreover, the normal-conducting region 101a formed in the initial stage of the current limiting operation is only a part of the whole superconductor 101. The resistance Rtot of the superconductor 101 after it is totally converted into a normal conductor meets the following condition:R<<Rtot  (4).Therefore, a combined resistance Rel of the whole current limiting element is expressed by the following equation (5):
                              R                      e            ⁢                                                  ⁢            l                          =                                            r              ⁢                                                          ⁢              R                                      (                              r                +                R                            )                                ≅                      r            ⁢                                          <<                                  R                                      t                    ⁢                                                                                  ⁢                    o                    ⁢                                                                                  ⁢                    t                                                              .                                                          (        5        )            
That is, when the structure of FIG. 1 is adopted, the resistance of the whole current limiting element is remarkably low as compared with the resistance Rtot of the superconductor. For example, in the case of using a Y-based superconducting thin film, it is known from a result of many experiments that the resistance r of the normal conductor 102 must be reduced by about one digit as compared with the resistance Rtot of the superconductor 101. Thus, in order to produce a desired resistance value for the current limiting operation, it is necessary to connect a large number of current limiting elements having the structure of FIG. 1 in series.
Under the circumstances, methods to make the superconducting-to-normal transition cause uniformly with preventing the superconductor from being damaged in the initial stage of the current limiting operation has been studied as to a structure having no normal conductor or as to a structure in which a normal conductor of a high resistance is connected in parallel with the superconductor. Representative examples include a method of applying a magnetic field to the superconductor and a method of raising temperature of the superconductor using a heater so as to lower superconducting characteristics and cause the superconducting-to-normal transition in a wide area. However, these methods require a coil for generating the magnetic field or a heater. Moreover, these methods also require an overcurrent sensor for determining whether the coil or the heater is turned on and a power supply for supplying power to the coil or the heater. Therefore, it is difficult in these methods to make the current limiting element compact.
A current limiting element has also been proposed in which the current of the electric power system is supplied to a heater, when a superconductor is quenched, to raise the temperature of the superconductor without providing any external power supply (see Jpn. Pat. Appln. KOKAI Publication No. 8-223790). The above current limiting element makes it easy to cause uniformly the superconducting-to-normal transition. However, the current is supplied to the heater as a result of voltage generation by the superconductor, i.e., the formation of the local normal-conducting region in the superconductor occurs prior to the supply of the current to the heater in view of time sequence. Therefore, the Joule heat in the local normal-conducting region needs to be reduced in order to prevent the superconductor from being damaged. Consequently, the resistance of a resistor used as the heater connected in parallel with the superconductor needs to satisfy the relation of the above equation (3). That is, the above current limiting element can reduce a time required for the whole superconductor to be converted into a normal conductor, but it cannot increase the resistance.